Lateral flow assays are the most frequently used formats for immunosensors, such as, for example, the pregnancy test in urine, or the drug test in saliva. There, the flow is created by a piece or sheet of paperlike material, like nitrocellulose, microporous nylon, etc. The microporosity of the sheet material creates a strong capillary action without the necessity of micromachined parts. It can operate in an open system and in large thickness and, thus, big volume. Flow rates are determined by the capillary force, which is inversely proportional to the pore size, and the effective aperture, which depends on the cross-section of the material perpendicular to the flow direction and the filling ratio or the porosity. The flow resistance scales with the inverse of the square of the pore size at constant aperture. With decreasing pore size, the resistance increases stronger than the capillary force, so that the apparent flow rate decreases. In a typical arrangement of a lateral flow assay device, different zones are present for the different functions, and different porous materials are used in the different sections.
From WO 2006/054238 A2 (US 2008/0185043A1), a microfluidic device for guiding the flow of a fluid sample is known that comprises a base plate that extends in two lateral directions and has a least one through-going recess in the vertical direction, a flow-through unit that has at least a first and a second flow-through site, and a plate structure. The flow-through unit is arranged relatively to the recess of the base plate so that a vertical fluid flow from one side of this arrangement to the opposite side through each of the first and the second flow-through sites and through a linking channel cavity formed between the flow-through unit and the plate structure is enabled. In the plate structure, an active component such as a sensor, an actuator or a pump may be integrated.